1. Technical Field
The present invention relates to a method for forming thin film transistors, and in particular to a method for forming oxide thin film transistors.
2. Related Art
Thin film transistor display devices such as thin film transistor liquid crystal displays (TFT LCD), electrphoretic displays (EPD) and organic light emitting diode displays (OLED) are widely employed in various electronic applications, for example, small size applications such as mobile phones, and great size (e.g., 40 inches) television sets. Thus, the studying and development of the structure and manufacturing process of thin film transistors are always of concern to people.
Conventional technique of employing amorphous silicon (a-Si) as the core of the transistors can't meet the requirements in some fields. For example, the electron mobility of a-Si is generally less than 1 square centimeter per volt second (cm2/V·sec). However, in applications that require high electron mobility such as active-matrix organic light-emitting diode (AMOLED), the electron mobility should reach to or greater than 2 cm2/V·sec. For current leakage, referring to FIG. 1, which is a scatter diagram showing the voltage-current (Vg-Id) characteristic curve of a transistor applied in a conventional TFT LCD, it is illustrated that the leakage current is generally between 10−12 amperes (A) and 10−14 A, and is difficult to be less than 10−14 A. In addition, in order to simplify the manufacturing process and the structure, the technique of directly integrating other circuit member and functions on the thin film transistors substrate is developed out. However, conventional a-Si is also not competent for such applications. Besides, the stability of a-Si is low and is not easy to meet the standards of mass production.
Low temperature poly-silicon (LTPS) can overcome aforementioned problems, however the manufacturing process of LTPS is hard and the yield rate is low, and thus it is difficult to apply LTPS in commercial products.
Oxide transistors have the advantages of high electron mobility and high stability, and can solve above problems. However, because the oxygen contained in the oxide semiconductor, a redox reaction easily occurs in the manufacturing process. Thus, there is a desire to provide a manufacturing process that can prevent the redox reaction of the oxide semiconductor and the obtained transistors have the advantages of low current leakage and high electron mobility.